"This method reports the development of a direct analysis method for edible oil samples using GFAAS without digestion. The advantages of using this method include small sample volume, direct introduction of samples, high sensitivity, and rapid analysis times. The application of GFAAS to arsenic, lead and cadmium analysis in edible oils was performed. The optimal pyrolysis and atomization temperatures, limit of detection, quality control (QC) checks and recoveries were studied in order to develop a rapid and accurate method. Learn more about our solutions: http://bit.ly/1dqSYtg

1. a p p l i c at i o n N o t e
Atomic Absorption
Author
Praveen Sarojam Ph.D.
PerkinElmer, Inc.
Global Application Center
Mumbai
Trace Elemental
Characterization of
Edible Oils with Graphite
Furnace Atomic Absorption
Spectrophotometer
Introduction
The determination of the inorganic profile of oils is
important because of the metabolic role of some
elements in the human organism. On the one hand,
there is knowledge of the food's nutritional value,
which refers to major and minor elements. On the
other hand, there is the concern to verify that the
food does not contain some minerals in quantities
toxic for the health of the consumers, regardless
whether this presence of minerals is naturally
occurring or is due to contamination during the
production processes. Oil characterization is the basis for further nutritional and food technological
investigations such as adulteration detection1. The most common adulteration is an addition of a
cheaper vegetable oil to expensive oil. Authenticity is a very important quality criterion for edible
oils and fats, because there is a big difference in prices of different types of oil and fat products.
Adulteration detection is possible by determining the ratio of the contents of some chemical
constituents and assuming these ratios as constant for particular oil. In regard to adulteration
detection, approaches based on atomic spectroscopy can be attractive2. The quality of edible oils
with regard to freshness, storability and toxicity can be evaluated by the determination of metals.
Trace levels of metals like Fe, Cu, Ca, Mg, Co, Ni and Mn are known to increase the rate of oil
oxidation. Metals like As, Cd, Cr, Se etc. are known for their toxicities. The development of rapid
and accurate analytical methods for trace elements determination in edible oil has been a challenge
in quality control and food analysis. However, sample pretreatment procedures are required in order
to eliminate the organic matrix. These include wet, dry or microwave digestion, dilution with organic
solvent and extraction methods3. The content of metals and their species (chemical forms) in edible
seed oils depends on several factors. The metals can be incorporated into the oil from the soil or be
introduced during the production process. Hydrogenation of edible seed oils and fats has been

2. performed using nickel catalysts. The presence of copper
and iron can be caused by the processing equipment as
well. Different digestion methods were applied for oil
digestion prior to spectrometric measurements. Many of the
used wet or dry digestion methods are not recommended
for use in high fat material because of the associated safety
hazards. GFAAS is a suitable and widely used technique for
the trace level determination of metals due to its selectivity,
simplicity, high sensitivity, and its capability for determination
in various matrices.
This paper reports the development of a simple method for
the analysis of edible oil samples by using Graphite Furnace
Atomic Absorption Spectrophotometer (GFAAS). Sample
preparation has been done by using a microwave digestion
system. Metals like Fe, Cu, Mn, Ni, Cr, As, Cd, Pb, Se and
Zn were analyzed using the developed method.
Experimental
The measurements were performed using the PerkinElmer®
AAnalyst™ 800 Atomic Absorption Spectrophotometer
(PerkinElmer, Inc., Shelton, CT, USA) (Figure 1) equipped
with WinLab32™ for AA Version 6.5 software, which features
all the tools to analyze samples, report and archive data and
ensure regulatory compliance. PerkinElmer high efficiency
double beam optical system and solid-state detector provide
outstanding signal-to-noise ratios. The AAnalyst 800 features longitudinal Zeeman-effect background correction for
furnace and the solid-state detector which is highly efficient
at low wavelengths. The AAnalyst 800 uses a transversely
heated graphite atomizer (THGA) which provides uniform
temperature distribution across the entire length of the
graphite tube. This eliminates the memory effect inherent
with the high matrix sample analysis. The THGA features
an integrated L’vov platform which is useful in overcoming
potential chemical interference effects common to the
GFAAS technique. EDL lamps were used whenever available.
Figure 1. PerkinElmer AAnalyst 800 Atomic Absorption Spectrophotometer.
2
A Multiwave™ 3000 Microwave system (PerkinElmer/
Anton-Paar) was used for the microwave-assisted digestion.
This is an industrial-type oven which is equipped with
various accessories to optimize the sample digestion.
The samples were digested in the Rotor 8XF100 in eight
100 mL high pressure vessels made of PTFE-TFM protected
with its individual ceramic jackets. TFM is chemically modified
PTFE that has enhanced mechanical properties at high
temperatures compare to conventional PTFE. This vessel has
a “working” pressure of 60 bar (870 psi) and temperatures of
up to 260 ˚C. A Pressure/Temperature (P/T) Sensor Accessory
was also used for this work. The P/T sensor simultaneously
measures temperature and pressure for one vessel. All vessels’
temperatures were monitored with the IR Temperature
Sensor Accessory. This measures the temperature of the
bottom surface of each vessel liner remotely during the
digestion process, thus providing the over-temperature
protection to each vessel.
Figure 2. PerkinElmer/Anton Paar Multiwave 3000 Microwave Digestion
System.
Standards, Chemicals and Certified Reference
Material
PerkinElmer single element calibration standards for Atomic
Spectroscopy were used as the stock standards for preparing
working standards. All the working standards were prepared
daily in Millipore® water (18.2 MΩ cm) acidified in 0.2%
Suprapur® nitric acid. Suprapur® nitric acid used for preparing
the diluent for standards was from Merck® (Darmstadt,
Germany). Chemical modifiers were prepared from stock
solutions, by diluting with acidified Millipore® water and
were added automatically to each standard, blank and sample
by the autosampler AS 800, an integral part of the AAnalyst
800. Standards were prepared in polypropylene vials
(Sarstedt®) and were prepared on volume-by-volume dilution.
Micropipettes (Eppendorf®, Germany) with disposable tips
were used for pippetting solutions. Certified Reference
Standard for trace metals in soybean oil from High Purity
Standards (Lot # 0827322) was used for quality control.
Multi-element ICP standard for trace metal ions in nitric
acid from Spex Certiprep®. (New Jersey, USA), prepared at
midpoint of the calibration curve was used as quality control
check standard.

3. Sample Preparation
Three common edible oils: coconut oil, sunflower oil and
soybean oil were bought from a local supermarket and were
used without any pre-treatments. ~0.25 g of each sample,
accurately weighed in duplicate was transferred to the
digestion vessels of the microwave digestion system and the
sample digestion was done in accordance with the program
given in Table 3.
The digested samples were diluted with 0.2% HNO3 and
made up to 25 mL in polypropylene vials. Plastic bottles
were cleaned by soaking in 10% (v/v) HNO3 for at least 24
hours and rinsed abundantly in de-ionized water before use.
The instrumental conditions for furnace experiments are
given in Table 1, and the graphite furnace temperature
programs are listed in Appendix I. A heated injection at 90 ˚C
was used for all the experiments. Pyrolytically coated graphite
tubes with integrated platforms were used. The autosampler
cups were soaked in 20% nitric acid overnight to minimize
sample contamination, and thoroughly rinsed with 0.5% HNO3
acid before use. Five point calibration curves (four standards
and one blank) were constructed for all the metal ions and
the calibration curve correlation coefficient was ensured to
be better than 0.999 before the start of the sample analysis.
Dilution of the order of 1000-10000 were required depending
up on the metal ion analyzed.
Results and Discussion
The goal of this method development was to develop a
simple method for the quantitative analysis of various toxic
metals and other trace metals in edible oils. The validity of
the developed method has been ensured by incorporating
various Quality Control (QC) checks and analysis of Certified
Reference Material (CRM).
The QC samples gave excellent recovery of between
94-111%. Excellent spike recovery for these QC samples
ranging between 91-109% was also achieved. The recovery
for the reference soybean oil was between 91-107%.
Method detection limits (MDLs) were calculated based on
the standard deviation of seven replicates of the samples
(student t-value of 3.14 for a confidence interval of 98%).
The excellent detection limits obtained shows the capability
of AAnalyst 800 in analyzing difficult matrices at lower
concentrations. The calibration curves obtained had correlation coefficient as good as 0.999 for all the metal
ions under study.
Conclusions
A simple method for the sequential quantitative determination of trace metal impurities in edible oil samples was
developed. The patented THGA tube used in the AAnalyst
800 provides a uniform temperature distribution along its
entire length. This eliminates cooler temperatures at the
tube ends and removes most interference. There is no
re-condensation, carry-over and memory effect is eliminated.
With the THGA tube design, accuracy and sample throughput
are improved by reducing the need for the time-consuming
standard additions technique. With the longitudinal Zeemaneffect background correction, the amount of light throughput
is doubled by eliminating the need for a polarizer in the
optical system. All other commercial Zeeman designs incorporate
inefficient polarizers that reduce light throughput and diminish
performance. With this unique design, the AAnalyst 800
provides the lowest detection limits available.
In conventional furnace systems, the heating rate during
atomization depends on the input-line voltage. As voltage
may vary from day to day, season to season or among
laboratory locations, so may the heating rate. The highperformance AAnalyst 800 uses enhanced power control
circuitry to maintain a uniform heating rate, irrespective
of the location of the instrument, one can be sure that it
provides outstanding, and consistent performance.
The AAnalyst 800 atomic absorption spectrophotometer also
produces highly accurate, fast and reproducible results with
difficult matrices such as edible oil. The developed method
has been validated by using reference material and the
method has been successfully applied for the analysis of different edible oil samples.
The Multiwave 3000 microwave digestion system has proven
to be an excellent tool for digesting difficult matrices such
as edible oils.
3

4. Table 1. Experimental Conditions of AAnalyst 800.
Element
Cd
Pb
As
Se
Ni
Cu
Fe
Mn
Cr
Zn
Wavelength
(nm)
228.8
283.3
193.7
196
232
324.8
248.3
279.5
357.9
213.9
Slit (nm)
0.7
0.7
0.7
2.0
0.2
0.7
0.2
0.2
0.7
0.7
Mode
AA-BG
AA-BG
AA-BG
AA-BG
AA-BG
AA-BG
AA-BG
AA-BG
AA-BG
AA-BG
Calibration
Linear w/
Calc. int.
Linear w/
Calc. int.
Linear w/
Calc. int.
Linear w/
Calc. int.
Linear w/
Calc. int.
Linear w/
Calc. int.
Linear w/
Calc. int.
Linear w/
Calc. int.
Linear w/
Calc. int.
Linear w/
Calc. int.
Lamp
EDL
EDL
EDL
EDL
HCL
HCL
HCL
HCL
HCL
HCL
Current (mA) 230
440
380
280
25
15
30
20
25
15
Standards
(µg/L)
0.5, 1.0
1.5, 2.0
20, 30
40, 50
20, 30
40, 50
25, 50
75, 100
20, 30
40, 50
5, 10
15, 25
5, 10
15, 20
2.5, 5
7.5, 10
4, 6
8, 10
0.4, 0.8
1, 2
Correlation
coefficient
0.999831
0.999799
0.999656
0.999789
0.999735
0.999533
0.999035
0.999581
0.999707
0.999972
Spike (µg/L)
1.0
10
10
10
5
2.5
5
1.5
4
0.5
Read time
(sec)
5
5
5
5
5
5
5
5
5
5
Measurement Peak Area
Peak Area
Peak Area Peak Area Peak Area
Peak Area
Peak Area
Peak Area
Peak Area
Peak Area
Injection
Temp (˚C)
90
90
90
90
90
90
90
90
90
90
Sample
Volume µL
20
20
20
20
20
20
20
20
20
20
Matrix modifier
0.05 mg
NH 4H2PO4
& 0.003 mg
MgNO3
0.05 mg
NH4H2PO4
& 0.003 mg
MgNO3
0.005 mg
Pd and
0.003 mg
MgNO3
0.005 mg Nil
Pd and
0.003 mg
MgNO3
0.005 mg
0.015 mg
Pd and
MgNO3
0.003 mg
MgNO3
0.005 mg
Pd and
0.003 mg
MgNO3
0.015 mg
MgNO3
0.005 mg
MgNO3
Modifier
volume µL
5
5
5
5
5
5
5
5
0
5
Table 2. Program used for Edible Oil Digestion with MDS.
Sequence
Ramp Time (min.)
Hold Time (min.)
Fan
1
600
5
2
1
2
900
5
2
1
3
1400
15
20
1
4
0
0
15
3
Weight Taken
~250 mg
HNO3
5 mL
H 2O 2
3 mL
Rate
0.3 bar/sec
HCl
4
Power
1 mL
Pressure
55 Bar

5. Table 3. Results of Edible Oil Analysis.
Metal
Coconut Oil I
µg/g
Coconut Oil II
µg/g
Sunflower Oil I
µg/g
Sunflower Oil II
µg/g
Soybean Oil I
µg/g
Soybean Oil II
µg/g
Pb
<DL
<DL
<DL
<DL
<DL
<DL
Cd
<DL
<DL
<DL
<DL
<DL
<DL
Cr
0.40
0.43
1.1
1.4
0.39
0.42
As
<DL
<DL
<DL
<DL
<DL
<DL
Se
0.08
0.12
0.10
0.07
0.05
0.03
Zn
0.78
0.79
0.80
1.5
3.9
2.26
Cu
0.12
0.11
0.14
0.13
0.09
0.10
Mn
0.57
0.60
0.21
0.19
0.15
0.18
Ni
0.24
0.23
4.16
5.27
0.37
0.25
Fe
7.05
6.75
3.93
4.3
4.08
4.6
Table 4. Results of CRM Analysis (Lot # 0827322).
Table 5. Results of QC and Spike Recovery.
Metal
Certified Value (µg/g)
% Recovery
Metal
QC 1 (%)
QC 2 (%)
Spike Recovery (%)
Pb
100 ±1.0
100.3
Pb
104.6
102.9
99.9
Cd
Not present
–
Cd
102.0
105.6
100.1
Cr
Not present
–
Cr
101.8
99.0
101.6
As
Not present
–
As
100.6
103.1
109.4
Se
Not present
–
Se
104.3
102.5
97.7
Zn
100 ±1.0
94.0
Zn
97.3
97.3
111
Cu
100 ±1.0
96.1
Cu
96.0
98.7
93.5
Mn
Not present
–
Mn
100.7
104.6
91.2
Fe
100 ±1.0
91.1
Fe
96.1
94.6
102.4
Ni
100 ±1.0
107.3
Ni
109.1
101.8
101.8
Table 6. Method Detection Limits (MDLs).
Metal
MDL (µg/kg)
Pb
19.8
Cd
0.8
As
48.4
Se
167.9
5

6. Appendix I. Graphite Furnace Temperature Program.
Element
Step Temp ˚C
Ramp
Time (Sec)
Hold
Time (Sec)
Internal Gas
Flow (mL/min)
Gas Type
Se
1
2
3
4
5
110
130
1300
1900
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
Cd
1
2
3
4
5
110
130
500
1500
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
As
1
2
3
4
5
110
130
1200
2000
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
Cu
1
2
3
4
5
110
130
1200
2000
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
Ni
1
2
3
4
5
110
130
1100
2300
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
Fe
1
2
3
4
5
110
130
1400
2100
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
Pb
110
130
850
1600
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
Zn
1
2
3
4
5
110
130
700
1800
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
Cr
1
2
3
4
5
110
130
1500
2300
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon
Mn
6
1
2
3
4
5
1
2
3
4
5
110
130
1300
1900
2450
1
15
10
0
1
30
30
20
5
3
250
250
250
0
250
Argon
Argon
Argon
Argon
Argon

7. Appendix II. Examples of Typical Calibration Curves and Atomization Profiles.
References
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Congress, 29 Sept. - 3 Oct. 2004.
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